The subject of the not yet published German Patent Application No. P 33 33 755.1 is a special magnetic device of a system for nuclear spin tomography which is equipped with several magnet coils which surround an interior space suitable for receiving a body to be examined and which generate therein an at least largely homogeneous magnetic field. The coils are surrounded by an approximately cylindrical shielding device of ferromagnetic material which comprises at least four beam-like shielding elements of identical design which are arranged regularly distributed in the circumferential direction on a common cylinder surface and have at their end faces one disclike part each with a central opening having a predetermined radius with respect to the cylinder axis.
Nuclear spin tomography systems (nuclear magnetic resonance (NMR) systems) for medical diagnostics are generally known (see, for instance, European Pat. No. 21 535 A1 or DE-OS No. 29 21 252). Such systems comprise a magnetic device with an arrangement of normal- or, in particular, super-conducting field coils by which a base field as strong as possible can be produced which has sufficiently high homogeneity in a measuring range. This base field is superimposed by pulsed gradient fields. In addition, a high-frequency field which is aligned perpendicularly to the base field must be provided. The dimensions of the corresponding coils must then be adapted to the dimensions of the body to be examined in such a way that the latter can be placed into the measuring range within an interior space surrounded by coils without difficulty. The strong magnetic field of such a coil arrangement is supposed to be substantially only in the measuring range but should if possible not spread into the surroundings of the coil arrangement in the form of a stray field, where it can lead, for instance, to disturbances in certain electromagnetic equipments or can also exert an undesired force action on movable ferromagnetic parts. Magnetic apparatus for nuclear spin tomography therefore comprises advantageously, besides the coil arrangement, also a special shielding device in order to keep the stray field limited outside the coil arrangement.
With the shielding device proposed by the mentioned patent application, a far-reaching return of the magnetic flux around the coil arrangement of the magnetic apparatus can be assured. Thus, the stray field of, for instance, a one-Tesla magnet can be reduced with such a device by a shielding factor of about 4 to 5. Sources of a magnetic stray flux are, however, mainly the central openings in the disc-shaped end plates at the end faces of this shielding device, where the opening diameter is fixed by the requirement of field homogeneity to a predetermined radius of, for instance, about 1.65 m and can therefore not be decreased as desired.